Energy Vanguard Blog

3 Problems with Fiberglass Batt Insulation in Floors

Posted by Allison Bailes on Tue, Mar 22, 2011

Fiberglass batt insulation is a poor choice because of its inability to do the job.

I've been in a lot of crawl spaces and basements that have floors insulated with fiberglass batts. I've seen a lot of new homes that have fiberglass batts installed in bonus room floors, cantilevers, and floors over basements or crawl spaces. Not once have I ever seen one and said, now that's how you insulate a floor!

Usually, I see one of these three problems:

  • Fiberglass batts falling down (above photo)
  • Fiberglass batts not in contact with subfloor (middle photo)
  • Fiberglass batts compressed (bottom photo)

Pretty much any crawl space with fiberglass batts in the floor above will have the first problem. It may not happen right away, but gravity is patient. Even though it's the weakest of the three fundamental forces of nature, it's plenty strong enough to pull those batts down over time. And it's relentless.

The second problem - batts not in fiberglass batt insulation not in contact with subfloorcontact with the subfloor - is usually present as well. Sometimes it's because the fiberglass batts aren't supported enough, so it makes contact at the supports (called tiger's teeth) but sags in between. Sometimes it's because they've gradually fallen over time. Sometimes the installers never put the batts in contact with the subfloor because they didn't understand its importance or it was too hard.

The third problem - compression - reduces the R-value of the insulation and is usually a result of the batts not being cut to the right size and not being fit properly around the obstructions (wires, pipes, ducts...). The photo below shows what is perhaps the best fiberglass batt installation I've ever seen in a floor system. It's still not even a Grade II, though, with Grade I being the best and Grade III being the worst. As I wrote last month, Version 3 of the ENERGY STAR homes program requires Grade I installation (with the exception noted below).

insulation fiberglass batt in framed floor system not Grade I

So, why does this matter? Well, would you like to get in bed on a cold night with the blanket on the floor? Or suspended a foot above your body? Would you install a new fridge and never plug it in? Would you buy a new car and have the mechanic disconnect half of the pistons?

When you get fiberglass insulation with the problems described above, it's the same. You're getting a product that's either completely ineffective, or it's compromised. One of the fundamental principles of building science is that a house needs a complete and continuous building envelope, with insulation and an air barrier that are in contact with each other.

In the case of fiberglass batts in floors, as I said above, I've never seen a Grade I installation. It's practically impossible to achieve, mainly because the tiger's teeth that hold the batts up compress the insulation even when they're otherwise installed perfectly, which they almost never are. Is it any wonder that the Green Curmudgeon suggested outlawing fiberglass batts?

Going back to ENERGY STAR Version 3, one of the new requirements is that insulation must be installed to Grade I or have continuous insulated sheathing and Grade II. I doubt builders are going to start installing foamboard on their floors, so that means framed floors have to be Grade I if they're part of the building envelope. I think builders are going to have to rethink the material they use to insulate floors or, even better, try to eliminate framed floors from the building envelope in the design phase.

Tags: ENERGY STAR, building enclosure, insulation